Filtering Body classes in pages according to taxonomy

I’m working on a plugin that modifies the titles of pages based on their hierarchy, page order, and a custom taxonomy.

If the page belongs has at least one term of this taxonomy, I want to modify the body classes, adding the slug for the taxonomy and the taxonomy term slug.

global $post;
		if (is_page()) {
			if(has_term('', 'book_page', $post->ID)) {
				$classes[]='book_page';
				$_terms = wp_get_post_terms($post->ID,'book_page');
				foreach ($_terms as $term) {
					$classes[] = $term['slug'];
				}
				return $classes;
			} else {
				return $classes;
			}
		} else {
			return $classes;
		}

This doesn’t pick up the presence of the taxonomy term. Am I using the has_term function correctly? Is there a better way to do this?